Methods for coding/decoding of speech signals in a radio communication system are well known and even standardized (for example IS-54 in the US and GSM in Europe). Furthermore, methods for improving encoding/decoding of background sounds, primarily intended for digital cellular telephone systems, have been described in Swedish Patent Application 93 00290-5. Both these types of methods are primarily designed to handle a situation where the connection between the encoder and the decoder is close to ideal, in the sense that only a small amount of bit or transmission errors remain after channel decoding. However, since the connection is a radio channel the received signal may contain some bit or transmission errors. Furthermore, frames may be lost for other reasons than a bad transmission channel. In for example the American digital cellular standard IS-54 the Fast Associated Control CHannel (FACCH) is established by stealing speech frames from the traffic channel (a similar channel exists in the European GSM specification). A similar situation arises in a packet switched network when packets (frames) are lost or arrive too late to be used for speech generation in real time (the packets may take different routes between sender and receiver). In all these cases it may be necessary to modify the methods mentioned above.
An object of the present invention is an apparatus and a method in which rejected frame concealment is applied to the received signal in order to make the decoding of audio signals more robust or insensitive to transmission errors and frame losses.